Internal combustion engines (e.g., vehicle engines, power generators, etc.), power plants, incinerators and other such combustion devices typically include exhaust systems which expel the products of a combustion process. Such exhaust systems typically include some form of flow-through catalyzing substrate and/or wall-flow filter substrate. These combustion products can include non-combusted and/or partially combusted byproducts such as, for example, soot particles, carbon monoxide, NOx, etc. Exhaust systems are typically designed so as to limit the release of such combustion byproducts into the atmosphere.
Particulate soot combustion byproducts have been found to pose health hazards to humans and the environment. As a result, the exhaust of such soot particles has received particular attention. In response to such concerns, increasingly strict governmental regulations have been and are being promulgated to restrict and reduce the exhaust emissions from sources such as internal combustion engines and, in particular, diesel engines. Therefore, additional attention has been directed toward the development of more efficient exhaust systems capable of further restricting and reducing the release of such combustion byproducts and, in particular, of filtering particulate laden exhaust gases.
A number of combustion devices (e.g., diesel engines) produce both undesirable gaseous (e.g., carbon monoxide) and particulate (e.g., soot) combustion byproducts. The exhaust systems of such engines are usually designed with a catalytic and a filter component for limiting the exhaust of both types of combustion byproducts. Catalytic converters typically include a flow-through catalyzing substrate that has a ceramic monolithic construction. Conventional flow-through catalyzing substrates are usually effective in furthering the combustion of exhaust gases (e.g., carbon monoxide to carbon dioxide); however, they are also relatively expensive and are not effective in combusting exhaust particulate. There are various commercially available wall-flow substrates for filtering particulate from exhaust gases. Such filter substrates include porous ceramic monoliths like that disclosed, for example, in U.S. Pat. No. 4,276,071. Such extruded substrates have been made from porous materials such as cordierite or silicon carbide. These extruded ceramic filters can be durable and effective, but they are also relatively expensive. Less expensive ceramic fiber-based particulate filters have also been made for this purpose but, to date, such filters have not exhibited the characteristics (e.g., durability and effectiveness) needed to achieve commercial success. See, for example, U.S. Pat. Nos. 3,112,184, 3,899,555, 4,608,361, 4,652,286, 4,718,926, 5,194,078 and 5,322,537. It is believed that the failure of such prior ceramic fiber-based filters to achieve commercial success has been due to a lack of durability in their intended working environment.